Potassium uptake is a fundamental property of cells. The polar epithelial cells of the renal tubule are capable of net reabsorption as well as net secretion of potassium depending on the nephron segment studied and the potassium balance of the animal. The mechanisms and factors controlling transepithelial potassium transport, however, remain only partially understood. In the proposed research the passive and active flows of potassium and their epithelial organization will be examined in the isolated urinary bladder of the water turtle, an epithelium of relatively simple structure in which the mechanisms of other electrolyte transport have been studied extensively. The turtle bladder resembles the distal nephron in its ability to transport Na+, Cl- and H+ against steep concentration gradients and in its response to hormones and diuretics. It is the objective of the research to define the transcellular and paracellular flows of potassium and to study the active and passive transport steps at the two cell membranes of the epithelium. The factors that determine the direction and magnitude of the net transport rate will be explored. Among the major factors to be analyzed are the electrical driving force generated by sodium transport, other forms of coupling to the transport of Na+, Cl- or H+, humoral control by mineralocorticoid hormone, insulin, cyclic AMP and the prostaglandins and the dependence of potassium transport on O2 and specific metabolic substrates. In addition, the effects on potassium transport of experimentally induced acid-base changes will be explored. Since the luminal cell membrane is directly accessible to translocator molecules for potassium, either inophores (valinomycin) or channel forming agents (nystatin, gramicidin), the effects of such translocators on transepithelial as well as transluminal potassium transport will be studied to assess the contribution of the luminal membrane to the overall epithelial handling of potassium.